1. Field of the Invention
The present invention relates to navigation of a path by a moving object, and, more particularly, to motion control systems for computer-controllable machine tools.
2. Description of the Related Art
Automation has resulted in the development of motion controllers capable of signaling actuator devices to effect motion in linkages along a desired trajectory while performing useful work. Motion controllers permit increased speed and precision in performing a given task over manual operation. Robots and automated manufacturing equipment are examples of a few of the products that utilize motion control technology. Programming these devices is often accomplished by specifying the desired trajectory as a collection of line/arc segments, along with the desired velocity of a tool along each segment. The velocity of the tool is often kept constant along each segment or group of segments of complex trajectories because velocity optimization along each point in the trajectory would be very time consuming.
Most humans who program tool trajectory have a fundamental understanding of the trade-off between velocity and accuracy. It is well known that at higher velocities it becomes more difficult for the control system to stay on the desired trajectory. Thus, trajectory programmers must make a trade-off between the velocity and the precision of motion along the desired trajectory. These decisions are often based on the programmer's experience and result in an iterative programming process wherein the trajectory is executed and then modified to reduce the velocity in sections where an undesirable deviation from the desired trajectory is observed. Thus, programmers control the deviation from the desired trajectory, and therefore the quality of the motion, by manipulating the tool velocity along the trajectory.
Motion control systems for manufacturing equipment, often referred to as Computer Numerical Controllers (CNCs), attempt to maximize the velocity of motion control while minimizing the deviation from the desired trajectory. CNCs may be used to control manufacturing equipment such as lathes, grinders and mills. CNCs are computing devices adapted for the real-time control of machine tools. A numerical controller receives a set of coded instructions that form a part program. Part programs are typically expressed in a standard G&M code language, or a close derivative of this language based on either the International Standards Organization (ISO) or the Electronics Industries Association (EIA) RS-274-D, using codes identified by letters such as G, M, or F. The codes define a sequence of machining operations to control motion in the manufacture of a part. The numerical controller converts the codes to a series of electrical signals which control motors attached to a machine tool effecting the motion of the tool along the programmed trajectory.
A motion controller operating a milling machine is one example of CNC. Lathes, grinders and coordinate measuring machines (CMMs) are other examples of manufacturing equipment which utilize a CNC for motion control. A three-axis CNC milling machine has a head where a tool is mounted, and a table movable relative to the tool in the X, Y plane. Motors control motion of the table in the X and Y directions and motion of tool in the Z direction, establishing an orthogonal X, Y, Z Cartesian coordinate system. Positional sensors (encoders or scales, typically) provide feedback indicating the position of the tool with respect to the coordinate system of the milling machine. The CNC reads in a part program specifying a tool path trajectory that the tool is to follow at a specified velocity or feedrate. The controller continuously compares the current tool position with the specified tool path. Using this feedback, the controller generates signals to control motors in such a way that the tool's actual trajectory matches the desired tool path or trajectory as closely as possible while the tool moves along the tool path at the desired velocity. The controller may be used in conjunction with a computer aided machining (CAM) system.
The deviation of the actual tool trajectory from the desired trajectory or tool path is referred to as “machining error.” The machining error may be computed as the distance between the instantaneous tool position and the desired trajectory as specified by the tool path. CNC tolerance is defined as the amount of the permitted machining error while machining. Motion controllers are expected to maintain good or tight CNC tolerance. The machining error depends on many factors including the performance of the motion controller and the feedrate selected for traversing the trajectory during machining. In general, higher feedrates will result in larger machining errors.
Known part programs do not explicitly address CNC tolerance issues. The machine tool operator, part programmer or machinist must set feedrates to attempt to address these issues. In fact, tolerance cannot be expressed using known CNC programming languages, such as EIA RS-274-D, nor do existing motion controllers support the notion of constraining motion so that a CNC tolerance specification is met.
A tolerance based motion control system, including a method for setting feedrates based upon tolerance, is disclosed in U.S. Pat. No. 6,242,880, which is incorporated herein by reference. While this patent represents a significant step forward in the art of motion control, refinements are needed in order to improve feedrates while still operating within the tolerance limits.